1. Field of the Invention
The present invention relates to a method for monitoring oxide film deposition, and more particularly to a method for monitoring high temperature oxide (HTO) film deposition in a vertical HTO furnace.
2. Description of the Related Art
In the semiconductor industry, the oxide film deposition process which is performed in a vertical furnace has several issues to be solved. For example, as shown in FIG. 1A, the oxide film deposition process is used to form the silicon oxide films 51, 53 of an oxide-nitride-oxide (ONO) stack layers 55 of a semiconductor device 50. There is a nitride film 52 between the oxide film 51 and the oxide film 53. The semiconductor device 50 can be a non-volatile memory device or a flash memory device. However, the thickness of the oxide films 51, 53 formed on wafers which are loaded on different locations of the vertical furnace would vary due to the characteristics of the vertical furnace. To solve this problem, the wafer loading sequence between the depositions of the top oxide film 53 and the bottom oxide film 51 of the ONO stack layers 55 is reversed to balance the variation of thickness of the ONO stack layer.
Since the thickness of the oxide film is a crucial dimension for the semiconductor device 50 such as a non-volatile memory device or a flash memory device, monitoring the oxide film deposition is inevitable. In order to monitor the deposition of the oxide film, monitor wafers are utilized to measure the growth rate of the oxide film deposition. However, the conventional monitoring method has a troubling drawback. As shown in FIG. 1B, the monitor wafers 100a, 100b and 100c having native oxide films 102a, 102b and 102c thereon are located on the top, middle and bottom locations inside a furnace 300 respectively. A plurality of process wafers 400 are loaded together into the furnace 300 to form oxide films. The native oxide films 104a, 104b and 104c are formed during the wafer loading process. The variation of the thicknesses of native oxide films 104a, 104b and 104c generally results from the air flow brought into the furnace 300 during the wafer loading process and the different locations inside the furnace 300. After the wafer loading process, the furnace 300 is closed and starts to grow oxide films 106a, 106b and 106c on the native oxide films 104a, 104b and 104c. Because the native oxide films formed on the monitor wafers 100a, 100b and 100c are not well controlled, the growth rate of the oxide film deposition on the monitor wafers 100a, 100b and 100c would mislead the setting of the oxide film deposition process and result in wrong thickness of oxide films 51, 53 of the semiconductor device 50 such as a flash memory device deviate the predetermined value and unstable electrical characteristics of the semiconductor device 50.
In view of the drawbacks mentioned with the prior art, there is a continued need to develop a new and improved method for monitoring oxide film deposition that overcomes the disadvantages associated with prior art. The requirements of this invention are that it solves the problems mentioned above.